Diamond is the hardest material known to man. Because of this, it finds extensive industrial application where ultra-hard material properties are needed. Due to its high hardness, it is difficult to make diamond tools of different shapes and sizes purely from cutting and shaping diamond. This has led to the development of diamond composite materials which consist of small diamond grains either sintered together through a liquid phase sintering process, or held together in a matrix by a binder phase material. The former process gives rise to the class of polycrystalline diamond materials (PCD), while the latter results in a number of composite materials, of which the foremost is that of SiC-diamond composites. The introduction of the second phase improves the formability and the fracture toughness of such diamond-based material.
Metallic phases such as cobalt are present in PCD and are commonly used as liquid phase sintering aids in the production of that material. These metals however were found to catalyse the graphitization of diamond thus limiting the application temperatures of these PCD materials to below 1000° C. Silicon carbide has been found to be exceptionally good as a diamond binder phase. Because of the structural similarities between diamond and silicon carbide, a strong bond forms between them that results in a material with very strong adhesion between the diamond grains and the SiC matrix. SiC is commonly formed in situ from the reaction between diamond and/or amorphous carbon or graphite with silicon. SiC does not react with diamond and hence the composite material can be used at temperatures above 1000° C. However, the application temperature may be limited by the melting temperature of silicon if some unreacted silicon is present in the final product.
There are two different generic routes of production of these composites:                mixing of a powdered silicon source with diamond particles and densification of the mixture under pressure with temperature (reaction sintering), or        infiltration by a silicon-containing melt of a preform made from diamond powder or from mixtures of diamond with graphite or resin.        
Reaction sintering to obtain fully dense compacts is only relatively straightforward under the high-pressure high-temperature (HpHT) conditions typically associated with diamond synthesis. Under low pressure conditions (such as Hot Pressing (HP) and Hot Isostatic Pressing (HIP)), the volume decrease associated with the local formation of SiC from the intermingled silicon source and diamond may well result in residual porosity. Therefore a pressure high enough for densification of the reacted compact such as diamond stable conditions can be necessary. This requirement for high or ultra high pressure limits the application of these materials due to production costs and the limited sizes and shapes accessible with this technique.
On the other hand, infiltration has been successfully utilised in generating fully dense composites even at low pressure conditions. This is explained by the fact that even if/as pores are generated within the structure during sintering, liquid phase is continuously wicked up from the infiltrant source to fill these pores. Effective infiltration therefore requires that the pores or channels in the preform structure remain open for infiltration. The limitation imposed by this pore size and density requirement means that infiltration has been chiefly employed for the manufacture of larger-grained diamond compacts, or those with a wide diamond grain size distribution. Even under HpHT conditions (7.7 GPa, 1400-2000° C.), infiltration of diamond powder with primary grain size of ˜10 nm but secondary particle (agglomerate) size of approximately 1 μm was only possible to a depth of 2 mm.
This pore retention problem is exacerbated by the ongoing formation of SiC within the preform. SiC formation from the interaction of molten Si infiltrant and the carbon source is accompanied by volume expansion of the solid phase. This reduces the size of the existing pore channels and can result in blockage thereof. This especially becomes a matter of concern for fine-grained preforms, which already have an extremely fine pore structure. An additional concern is that the formation of SiC is strongly exothermic, which further accelerates the reaction in a runaway effect.
Infiltration has a further advantage in that the purity of the silicon source can be more adequately controlled through the use, for example, of a monolithic silicon wafer. By contrast, a reaction sintering or admixing technique typically requires that a very fine powder be used in order to maximise microstructural homogeneity. This brings with it the associated impurities of high surface area particles, as well as concomitant contamination introduced during the preparative mixing or milling process.
A further issue in the generation of diamond-SiC compacts relates to the presence of free or elemental silicon in the final binder phase. The thermal stability of a compact containing discernible free silicon may be limited by the melting point of silicon, as the bond between diamond and binder phase can be compromised at this point. Typically the presence of free silicon is the mark of an incomplete reaction with the carbon source. This may occur where substantial SiC formation has masked or blocked off the silicon melt from carbonaceous material, as diffusion of these species through SiC is significantly slower than that along the grain boundaries
U.S. Pat. No. 4,124,401 describes a diamond compact comprising a mass of diamond crystals adherently bonded together by a silicon atom-containing binder. The compact is made by infiltration under relatively mild hot pressing conditions (<1 kbar), where pressure is applied to dimensionally stabilise the diamond mass before and during infiltration. The resultant binder comprises SiC and a further carbide and/or silicide of a metal component which forms a silicide with silicon. The diamond density of the compact ranges from 70-90 volume %. The metal component for the diamond body is selected from a wide group of metals such as cobalt, chromium, iron etc.
U.S. Pat. No. 4,151,686 describes a diamond compact similar to that of U.S. Pat. No. 4,124,401 save that the resultant binder comprises SiC and elemental or free silicon. The substantially pore-free compact is generated at significantly higher pressures (in excess of 25 kbar) through infiltration by an elemental silicon melt. These high pressures are required in order to achieve the characteristic high diamond density of the compact (from 80-95 volume %).
U.S. Pat. No. 4,664,705 discloses a method that infiltrates a silicon alloy through a previously intergrown polycrystalline diamond body, that was initially sintered in the presence of a transition metal solvent/catalyst, where this previous binder has been leached out. SiC forms in situ through the reaction of the molten silicon with the intergrown diamond at HpHT.
U.S. Pat. Nos. 6,939,506 and 7,060,641 describe the manufacture of fully dense diamond-SiC composites by reaction sintering at HpHT conditions (namely 5 GPa and temperatures between 600-2000° C.). The reagent mix is prepared by reactive ball-milling of diamond powder (5-10 μm particle size) and crystalline silicon powder. At higher sintering temperatures, the SiC binder that forms is nanocrystalline in nature; whilst at lower temperatures residual unreacted elemental silicon tends to remain in the binder phase. These compacts had a minimum possible calculated diamond content of 77 mass %. It was observed that ball-milling serves to transform the silicon to the amorphous state, which was critical in determining the nanocrystalline nature of the binder.
Another approach to the formation of SiC-diamond compacts is disclosed in U.S. Pat. No. 5,010,043 and associated applications. In a specific embodiment of this process, reaction sintering of a diamond-silicon mixture is employed together with silicon melt infiltration to form diamond-SiC compacts with a diamond density of 50-85 volume %. The silicon admixed within the compacts is postulated to melt and wet the surfaces of the diamond particles, establishing a continuous capillary system for infiltration. The compact formation conditions are intermediate between conventional HpHT and low pressure processes, at 10-40 kbar. Critical to this process is a deliberate plastic deformation step that is observed to significantly improve the properties of the resultant compacts and enable the use of p and T conditions reduced from those of HpHT. Given that it is known in the art that plastically deformed diamond is inherently more reactive than diamond which is not (see U.S. Pat. No. 6,680,914), it may be the case that the improved reactivity of the diamond in this invention is what enables effective bonding at lower p, T conditions. This is consistent with the fact that manipulation of the sintering temperatures generates compacts that contain minimal amounts of free silicon in the binder phase, as the SiC formation reaction has been maximised.
It is also known in the art to produce diamond-SiC compacts where the carbon source for the in situ SiC formation is not dominantly supplied by crystalline diamond but by a carbon introduced or produced on the diamond surface. Both low and higher pressure techniques employing this approach are known.
U.S. Pat. Nos. 4,220,455 and 4,353,953 describe diamond-SiC compacts formed by coating diamond particles with amorphous carbon before infiltrating under partial vacuum with molten silicon. The amorphous carbon is introduced by pyrolysis of organic binder systems such as resins, polymers etc., or by pyrolytic decomposition of carbonaceous gases. An advantage of the resin or polymer approach is that the organic residue can facilitate formability of the pre-sintered diamond. It was additionally observed that non-diamond carbon coatings were highly reactive in the presence of molten silicon, easily wet by it and hence easily formed SiC. However, the binder phase in these compacts still comprised both SiC and unreacted elemental silicon.
U.S. Pat. No. 4,381,271 employs carbonaceous materials such as fibrous graphite as an additional carbon source for SiC formation. These fibres are admixed with coated diamond particles before being infiltrated by molten silicon under a partial vacuum. In the final compact binder both SiC and unreacted elemental silicon were observed.
In most of these cases, any required pyrolysis is carried out to minimise the graphitisation of the diamond; as this is seen as detrimental to the potential properties of the compact. By contrast, U.S. Pat. No. 6,447,852 and associated applications disclose a low pressure infiltration process for the manufacture of diamond-SiC compacts that utilises a deliberate graphitisation step. Preferably 6-30 mass % of the diamond is deliberately graphitised prior to infiltration with molten silicon. It is postulated that the graphitised layer on the diamond surface affects the pore character such that an optimal infiltration environment results. A characteristic of compacts of this invention is the discernible presence of free silicon in the binder phase.
Infiltration remains a preferred method for the manufacture of diamond-SiC compacts because of the opportunity it provides for exploiting low pressure processes. There are significant cost benefits inherent in this approach over using HpHT; and further benefits of being able to access shapes and sizes not viably attainable in HpHT or even medium pressure processes. However, the use of infiltration for finer-grained diamond structures is problematic because of the fine-scale nature of the pore structure and the ease with which these pores can be blocked. Nonetheless, finer-grained structures would be of great interest as high performance composites. Additionally, the generation of a compact containing no discernible free silicon that uses a low pressure infiltration process would have significant cost and technical benefits.